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Carb tuning issues - 750 speed demon

30K views 48 replies 12 participants last post by  1985 427 monte ss 
#1 ·
I know I've read other posts about this, but every engine is different...so here we go.
When I was looking for a new carb I did a lot of homework. For my setup, a fairly well built up 351w with a 5 speed, the 750cfm speed demon, mech. secondaries, and annular boosters seemed to be the best choice. I am running a dual plane intake which should be helping my situation out. At idle I am getting a lot of smoke. Very rich condition. The intake smells of fuel after taking the carb off and after running the truck for a bit and cutting it off it still smokes. The problem seems to happen when it's at operating temps. I've been through the setup procedure over again to ensure I'm not missing anything, but my problem persists even after trouble-shooting. Just looking for some advise from some of the more experienced ones out there.
Thanks.
 
#2 ·
I also have a Holley 750 cfm double pumper, on a chevy 355. I had the same problem, my problem was the idle mixture, it was too rich. Make sure your engine is at operating temp. before setting it, a colder engine requires more fuel. If you have a big cam, the throttle plates will be open too far, rendering the idle mixture screws useless, i had to drill my throttle butterflies in order to get them closed enough. Besides the rich idle, do you have any other problems??
 
#3 ·
The demon has an idle-eze adjustment to make up for the big cam instead of the drilling the butterflies method. The problem is that I see an improvement in idle quality with this adjustment, but I still get smoke. I'm going to try some things out tomorrow so we will see. Other than the idle troubles I feel like the 750 demon gives me more giddy-up over the holley 670 avenger style carb. I definitely enjoy the mech. secondaries and I feel more top end power.
 
#4 ·
I am concerned about the air bleeds. I was told by a local shop to look into those being too open. My question is about the speed demons setup in that mine was shipped with no restriction on these air bleeds whatsoever. Does this make sense? Would that cause a rich condition? One thing to note is that at idle I hear the carb sucking in air through these and I was told this was a bad thing. I'm sure I'll get this problem behind me, but I am tired of getting conflicting information from the local boys..hah.
Thanks.
 
#6 ·
One of the best tools to use when setting any carb,especially a Demon is a vacuum gauge.I had all sorts of troubles getting mine set on the small block.When I jumped up to running a big block,I went back to the Ol Faithful Holley.

Concerning the sticking problem.Are you having this problem with the air filter on or off?I had the lower base on a Z28 of mine catching the linkage and keeping it revving higher.
 
#12 ·
If you have low vacuum at idle, usually because of cam specs, the power valve will be open at idle and make the engine very rich. Check your vacuum at idle then see what number power valve you have. On a Holley it's written on the power valve. I think I've been told the power valve should be 1/2 of your idle vacuum -2. For example if your idle vacuum is 16 the power valve should be a number 6. I may be a little off on that number, but I'm sure someone will correct me if I'm wrong......lol.
 
#15 ·
If you are right and the power valve should be 1/2 - 2...I have the wrong power valve. Right now the engine pulls 13hg and it has the 6.5 in it. Initial timing is set at 16*, but I've been trying the range from 10*-20*. Cam is about .550 lift and 220 duration at .050.
 
#16 ·
I just checked my Holley instruction manual and they recommend using a 45 power valve. My Holley carb was always loading up at idle and low speeds and switching to a 45 power valve cured my problems. I have a little less lift and a little more duration in my cam. I've been through the discussion with guys before about power valves not any difference at idle and I didn't think they did either. An expert at fine tuning carbs suggested I try it. When I took his advice and went to a lower numbered power valve my idle and off idle problems went away.
 
#17 ·
carb

ernkazern said:
Go back to school Keith

You better take some time and read a book your self because your clueless!!

It's not rocket science.,,,, The power valve is in the power circut of the carb and not in the idle circut. Holleys have always been this way. You can believe what ever you like but it is the truth...

If you switched a power valve and it fixed a rich idle it's because something else changed with it,not because it was the wrong number power valve..

Now if the power valve was ruptured it can bleed fuel into the main body side of the carb and cause a rich condition. Not because it feeds the idle circut but because it will feed raw fuel directly into the intake by way of the vacuum hole that operates the power valve. This is the only way it affects the idle.

If you would install a power valve that was open all the time it would not change the idle of the carb one bit, providing it's a proper functioning carb to begin with.

Keith
 
#19 ·
carb

You are correct i am no guru, but i have worked on a bunch of holleys over the last 25 years of engine building and i have picked up a few things along the way.

It's no "theory" that the pv feeds the main well/booster circut and not the idle circut. Go to you local book store tonight and find a book on holley carbs. They all have pictures of meetering blocks in them and they show how the pv circut works...Or go back to your carb expert that you asked to help you fix your problem and have him trace the pv circut out for you, and i gaurantee you will see what i am saying is true...

The pv feeds the main well in the meetering block, the main well is full of fuel at all times. In the bottom of the main well is the idle feed restriction. This restriction is were all holleys get the fuel for the idle circut. Adding more fuel to the main well cannot effect the amount of fuel passing through this restriction. Unless the main well is empty, then it's another carb issue. It would be like saying if i have a full tank of gas my carb will get more fuel then if i have a half tank of gas. The amount of fuel in the tank has nothing to do with the amount of fuel the carb recieves, that is regulated by the pump,line size, etc...

If what your saying was true than the 1000's of 4 corner idle carbs that have the rear pv blocked would not be 4 corner carbs they would be 2 corner carbs...Again it's just not true.

If you replaced a pv and it fixed a rich idle on your engine, then it's because something else changed along with the pv, unless like i stated the old pv was ruptured.

I would suggest before you get on here and start attacking someone on a personal level you have a good come back to your theory, and not what worked for you 1 time, and you even had to ask someone else what to fix to correct the problem that time...

Keith
 
#20 ·
lightningcow said:
If you are right and the power valve should be 1/2 - 2...I have the wrong power valve. Right now the engine pulls 13hg and it has the 6.5 in it. Initial timing is set at 16*, but I've been trying the range from 10*-20*. Cam is about .550 lift and 220 duration at .050.

Do you have a vacuum advance on your distributor?? If so are you setting the timing with it disconnected???

First thing that we need to do is figure out if the carb has any internal leaks, causing the rich idle or if it's a function of the carb.

Have you checked the float level??? and fuel PSI?
Once you determine that the fuel PSI is correct and the floats are set right. You need to install a new mounting gasket, while the carb is off look at the idle transfer slots, they should not have more then .020/.030 exposed. Install the new gasket and run the engine up to temp and or go for a short ride. remove the carb carefully not bumping the accel pumps and see if the gasket is wet..

Let us know how that all works out and we will go from there.

Have you taken the carb apart at all???

Keith
 
#23 ·
If you scroll down to the bottom of the page you will find a couple of topics about Demon vs. Holley. A guy lists his cam and it is a very healthy solid lifter cam with about 250 duration @.050 and over .520 lift. Tech BG said that for that cam they would recommend a 750 Demon! Is your cam that healthy? If not, you are over-carbed! Demons are different than Holleys and if you don't match up the Demon to your specific engine parameters by Demon's guidelines you can run into trouble. Have you called the tech department at Grant? If not, what are you waiting for? :cool:
 
#24 ·
The carb is matched just fine...it would be an issue of jetting to get more fuel in with all the air the 750 is pulling in. I've called the tech dept more than a few times and it sounds like the Twix commercials where the guy crams one in his mouth so he doesn't have to give a straight answer. They usually give me the same information the instruction manual tells me.
 
#25 ·
carb

When you were in the carb did you happen to notice if the main body side of the primary meetering block was wet with fuel??? it should be bone dry.

I would strongly suggest the addition of a vacuum advance. We can make that call for sure once you let us know how much of the transfer slots are exposed.

Keith
 
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